1. Field of the Invention
The invention relates to a magnetic yoke for an induction crucible furnace, having a barlike lamination packet which is suitable for guiding magnetic flux generated by the furnace coil of the induction crucible furnace and which includes a number of individual single laminations being electrically insulated from one another.
Such a magnetic yoke for an induction crucible furnace is known from a publication entitled ABB-Druckschfirt [ABB Publication]No. D ME/D 118289 D. The induction crucible furnace is suitable for inductive melting of cast iron, steel, light metal, heavy metal and alloys. When constructed as a medium-free induction crucible furnace, its operation takes place at frequencies of 125 to 1000 Hz, for example. A power converter is used to establish an alternating voltage at a given frequency.
The active part of the induction crucible furnace is the furnace coil which has an interior that sheathes a ceramic crucible. The alternating current flowing through the furnace coil produces a magnetic alternating field, which is carried through the metal starting material (melt) inside the furnace crucible and is carried through the iron lamination packets of the magnetic yokes outside the coil.
The magnetic alternating field induces eddy currents in the metal starting material, or in other words electrical energy that is converted into heat. According to the transformer principle, the furnace draws power from the power supply, so that with energy being delivered continuously, the starting material is made to melt. The electromagnetic forces acting upon the melt cause an intensive motion in the bath, which assures a rapid equalization in terms of heat and mass.
The magnetic yokes are disposed on the outside of the coil, in the form of individual single packets that are distributed parallel to the furnace axis over the periphery of the coil, with interstices between them. Each single packet includes a number of thin transformer laminations being electrically insulated from one another and having a high specific electrical resistance and high permeability. The iron lamination packets of the magnetic yokes serve the purpose of carrying the magnetic alternating flux, as already noted above. The intent is to afford the magnetic flux a path of low magnetic resistance, which at the same time causes only slight eddy current losses. Due to the use of the magnetic yokes, as a consequence of the reduction in magnetic resistance in the yoke region of the flux, the unavoidable reactive power is lessened. At the same time, the flux is kept from entering the usually ferromagnetic, load-bearing outer components of the furnace (furnace body with lining), thereby preventing its being heated by eddy currents.
It has been found that aside from the eddy current losses, which are caused by the magnetic alternating field extending predominantly parallel to the laminations, additional, sometimes considerable locally limited eddy current losses also occur at certain points of the lamination packets. In the interstice between the oven coil and the melt and also in the region of the penetration depth of the magnetic alternating field into the melt, the magnetic resistance is constant along the coil periphery, or in other words in the azimuth direction. Accordingly, the flux densities along the coil periphery are also constant, and the field lines all run parallel to the furnace axis.
Conversely, in the above-described configuration of the magnetic yokes on the coil periphery, regions of low magnet resistance alternate with regions of high magnetic resistance (lamination packets and interstices) in the yoke space of the field on the outside of the furnace coil. For the flux, regions of high magnetic conductance are accordingly connected parallel to those of very low conductance. Thus in the outer region of the coil, the flux finds its way largely through the regions of high conductance, or in other words it is carried virtually exclusively in the lamination packets.
However, at the upper and lower ends of the coil or lamination packet it propagates not only radially in the direction of the middle of the crucible (normal field), but also circumferentially of the coil, or in other words largely horizontally (transverse field), and then the interior of the coil changes into a circumferentially uniform flux density. In other words, some of the flux at the end region of the lamination packets emerges from the packets transversely to the plane of the laminations. As a result, considerable additional eddy current losses in the end region of the lamination packets are produced, which can lead to local overheating of the lamination packets and cover plates. If the power is high enough, then separate, expensive additional cooling provisions are necessary at those points.
It is accordingly an object of the invention to provide a magnetic yoke for an induction crucible furnace, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and in which spot induction at high specific power is prevented.